As discussed on IRC session tonight, a "Camera" button should be added
to the GTK frontend to take screenshots in PNG format.
Screenshots will be stored with names like
"debian-installer_main-menu_nnn.png" and a popup windows should inform
the user the screenshot has been saved correctly.
With this approach also users that haven't the PrtSc key would be able
to take screenshots in the common PNG format and the "Camera" button is
self-explicative too.

A "Screenshot" button has eventually been added to the GTK frontend: the
camera image was ATM unapplicable to the button since the required
gtk_button_set_image() function is available starting from GTK v 2.6 only.
The screenshot is saved as /var/log/screenshot_<question tag, with "/"
changed to "_">_n.png and a popup message appears to the user and tells
him "Screenshot saved as <name of the saved screenshot>".
This string should be localized, so i suggest adding a string into
cdebconf-gtk-udeb.templates like this.

I don't know if a string is needed for the "Screenshot" button too,
since i hope we'll be soon able to switch to GTK 2.8.x so that the label
can be replaced by the image camera.
Also, the httpd web server in installation-report package has been
updated to produce PNG headers for PNG screenshots and to sort files so
that all screenshot_*png files are grouped torgheter.
Also, i think allowing the user taking screenshots via DFB's PRTSC
function too is still useful, so that issues like bad pointer rendering
(see bug #341770) can be graphically documented.